I think it depended on engine and axle
Here's some info from Dale's Cuda Shop:
1970-71 E-BODY WITH 340, 440 & HEMI REAR AXLE BUMPER.
With 3 3/4" spacing and 9 3/4 Axle
PART #ZRAB2
Made In The USA
Replaces Chrysler Part Number 3400841
1971-72 E-BODY REAR AXLE BUMPER.
With out 340 3 2bbl and 9 3/4 Axle
PART #ZRAB5
Made in the USA
Replaces Chrysler Part Number 3400843